Reduction of uranium oxide



United States Patent 3,119,686 REDUCTION (if? URANEUM OXIDE Robert J. Teitel, Midland, Mich assignor to The Dow Chemical Company, Midland, Mich, a corporation oi Delaware No Drawing. lFiied Aug. 17, E960, Ser. No. 50,094 5 Claims. (Cl. 7584.1)

The invention relates to the reduction of uranium oxide with magnesium and more particularly relates to an improvement in the separation of the reaction products, uranium and magnesium oxide, one from the other, in zinc alloy medium.

Heretofore magnesium and magnesium-base alloys have not been widely employed in the direct reduction of uranium from its oxides. because of the ditliculty of separating the reaction products, uranium and magnesium oxide, one from the other. Both uranium and magnesium oxide are insoluble in magnesium. Thus there is no ready basis for separating the two products without the addition of yet another component.

In attempts to utilize the reducing action of magnesium, zinc has been alloyed therewith and the resulting zinc alloy heated together with uranium oxide. To separate magnesium oxide formed in the reaction and widely dispersed in the reaction product, a relatively large amount of the zinc alloy has been employed in order to dissolve the uranium therein while the alloy is molten. The molten alloy is then tfiltered under high temperature conditions to remove M-gO. Then the uranium content of the filtered alloy is precipitated as a uranium-Zinc intermctallic compound by reducing the temperature to a point below the previous filtration tempenature, but slightly above the solidfication temperature of the zinc. The precipitated uranium-zinc intermetallic compound is then separated by filtration, or, by settling, casting and sectioning.

The foregoing method of separating magnesium oxide necessitates the use of large quantities of zinc to dissolve the uranium in the presence of unreactcd magnesium, as well as filtration equipment suitable for high temperature operation, e.g., at 750 C., and the method is not Well adapted to continuous processing.

It is therefore an object of the invention to provide an improved method of separating magnesium oxide from the reaction mixture formed in the direct reduction of uranium oxide with magnesium or magnesium-zinc mixtures.

For the purpose of the specification and the appended claims the phrase magnesium-zinc mixtures is intended to include magnesium-Zinc alloys as Well as mixtures of pieces of the discrete metals.

Another object of the invention is to avoid high temperature filtration in the reduction of uranium oxide with magnesium or magnesium-zinc mixtures.

A further object is to provide a method of preparing uranium from uranium oxide by direct reduction of the oxide with magnesium-zinc alloy by a process which is adapted to continuous operation.

Still a further object of the invention is to provide an improved method of separating uranium from magnesium oxide with a Zinc metal or alloy by a process which re quires smaller quantities of zinc than used heretofore.

These and other objects and advantages of the method of the invention will be more clearly understood by those skilled in the art on becoming familiar With the following specification and the appended claims.

The invention is predicated on the discovery that upon providing at least 75 Weight percent of Zinc in the liquid phase of an intimate mixture consisting essentially of uranium, magnesium oxide, and a molten magnesiumzinc alloy containing at least 2 weight percent of magnesium and less than 60 parts by weight of zinc per part 3,1l9fi8b Patented Jan. 28, 1%64 "Ice of uranium, allowing the molten magnesium-zinc metal to stand quiescent for a time sufficient for the magnesium oxide to rise to the top portion of the mixture and the uranium to precipitate as a uranium-zinc compound, thereafter removing the top portion of the mixture from the remainder thereof, magnesium oxide is separated from the uranium which settled as the inter-metallic compound.

The separation process of the invention may be employed after the direct reduction of uranium oxide, c.g., U0 U 0 U0 and mixtures thereof, with magnesium metal. From about 5 to 50 parts by weight of zinc per part of uranium in the reduction reaction product, but at least 3 parts by weight of zinc per part of residual magnesium metal, are heated under an inert atmosphere together with the reduction reaction product to a temperature at which the zinc is molten and forms an alloy with the residual magnesium. Generally a temperature of 450 to 500 C. is suflicient for the magnesium-zinc alloy to be fluid enough for the magnesium oxide to rise. The mixture is agitated vigorously, as by stirring, and allowed to stand quiescent for a period, such as an hour.

During the quiescent period the magnesium oxide rises to the top part of the molten magnesium-Zinc alloy and uranium-Zinc intermetallic compound, such as UZn or U Zn forms and settles as a particulate solid. The top portion :of the molten magnesiurn-zinc alloy containing the magnesium oxide can be skimmed off or decanted, or the entire mixture can be allowed to cool and solidify as a casting and the layer containing the magnesium oxide cut off from the rest of the casting.

Uranium is recovered from the casting by cutting off the bottom portion containing the uranium-zinc intermetallic compound, then distilling the zinc and entrained magnesium from the uranium under an inert atmosphere.

In a more preferred manner of operation the reduction of uranium oxide with magnesium is carried out in the presence of Zinc. The zinc facilitates the intimate contact of the reactants by providing a reaction medium in which the products are dispersed and interfere less with the progress of the reaction.

Smaller proportions of zinc than those required for the separation process of the invention may be usefully added during the reduction reaction, and additional zinc added before separating magnesium oxide. However, it is generally convenient to add the requisite amounts of both the magnesium and zinc at the start.

To avoid incomplete reaction of the uranium oxide, it is best to employ somewhat more than the stoichiometric amount of magnesium, preferably at least percent of the stoichiometric amount, though larger amounts such as 20 times the stoichiometric amount may be used if desired, since unreacted magnesium in the magnesium-Zinc alloy is readily recycled in the process after the separation steps. In any event, the amount of unreacted magnesium in the mixture at the time the separation of magnesium oxide from the mixture is to be carried out should not exceed one-third of the weight of the zinc present.

The desired amount of zinc is, as indicated hereina'oove, about 5 to 50 parts by weight per part of uranium expected to be formed in the reduction.

Preferably the magnesium and zinc mixture does not contain more than about 5 weight percent of other metal inert toward uranium oxide, nor more than about 0.2 percent by Weight of other metal which is oxidizable by uranium oxide. In addition, metals of low volatility which readily form intermetallic compounds with uranium should be excluded from the magnesium-zinc mixture because of the difliculty of recovering uranium from such compounds.

The preferred reduction process may be employed in converting all uranium oxides, e.g., U0 U 0 U0 uranium ore concentrates, even those containing up to 1 s,119,ese

or 2 percent of iron and vanadium, spent uranium oxide fuel elements and the like, to uranium-Zinc intermetallic compounds, from which the uranium is readily recovered as uranium metal.

In carrying out the reduction process, uranium oxide, desirably in powder form, is heated together with a mixture of magnesium and Zinc under an inert atmosphere and at a temperature in the range of 650 to 950 C., preferably about 800 C.

The magnesium and zinc may be placed in a suitable reaction vessel in particulate or chunk form either as an alloy or as the discrete metals.

While the uranium oxide may be placed in the reaction vessel with the magnesium and zinc, or magnesium-zinc alloy, and the entire reaction mixture heated at once, it is usually preferable to form the molten alloy and then add the powdered oxide slowly and continuously over a period of time, such as an hour.

The reduction reaction is best carried out in equipment which is resistant to attack by a molten mixture of magnesium and zinc, e.g., a graphite crucible. Such a reaction vessel is provided with heating means such as an electric furnace in which an inert atmosphere can be maintained. Generally, agitation, as by stirring is also provided.

The reaction period varies somewhat with the quantities of reactants employed, the extent to which the reaction mixture is continuously or frequently mixed, and the reaction temperature. Uranium dioxide in 5 to gram quantities has been found to react substantially completely '90%) with the requisite amount of magnesium and zinc in about one hour at 800 C. upon providing sufficient agitation of the reaction mixture. Longer periods will be required with larger amounts.

During the reaction period, magnesium in the melt reacts with uranium oxide to form magnesium oxide, which is insoluble in molten magnesium-zinc, and uranium which combines with zinc to form one or more intermetallic compounds, such as UZn or U Zn which are likewise relatively insoluble in the melt. The uraniumzinc intermetallic compounds are relatively heavy and tend to settle out promptly while the magnesium oxide is relatively light and floats to the surface of the melt if the density of the reaction mixture is made great enough by employing at least 75 weight percent of zinc in the magnesium-zinc mixture.

At the end of the reaction period, stirring is stopped, and the reaction mixture is held quiescent for a time, e.g., 2 hours, to permit magnesium oxide to rise through the molten magnesium-zinc alloy. It is not necessary to keep the melt at temperatures much above the melting point thereof after the reaction has been completed. A temperature of 450 to 500 C. is usually adequate during the quiescent period.

At the end of the quiescent period, the upper layer of the melt, which now contains the magnesium oxide, is separated in one of several ways. The upper quarter or third of the melt may be decanted or skimmed off while still molten, or, the melt may be allowed to solidify as a casting and the upper section of the casting cut off. In a continuous process, the holding period can be reduced by increasing the settling rates by well-known techniques, such as centrifuging.

In any event, the settled uranium-zinc compounds are separated from the remainder of the melt as by allowing the melt to solidify as a casting. Then the bottom section of the solidified casting containing the settled uraniumzinc compounds is cut off. This uranium-rich section is then placed in suitable apparatus, such as a magnesia crucible in a graphite lined distillation furnace. Here the section is heated in an inert atmosphere under reduced pressure according to well-known distillation techniques. Intermetallic compounds are dissociated by heat and the magnesium and zinc content of the section distill away leaving only uranium. After the magnesium and zinc have been removed it is desirable to raise the temperature 4i of the furnace to somewhat above 1130 C. to fuse the uranium into a coherent mass since finely divided uranium tends to be pyrophoric when exposed to air.

Example As an example of the process of the invention, 5 grams of U0 17 grams of magnesium, and 250 grams of zinc were placed in a graphite crucible. The crucible and contents while under an inert atmosphere were brought to a temperature of 800 C. This temperature was maintained for an hour during which time the reaction mixture, which was in a fluid state, was frequently subjected to mixing. The temperature of the reaction mixture was then lowered to and held at 450 C. for about 2 hours, after which the entire mixture was solidified by allowing it to cool to room temperature. The solidified melt was sectioned transversely in horizontal planes relative to the position in which it solidified. Sections which were representative of the top, middle, and bottom portions of the solidified material were examined by chemical, metallographic, and X-ray diffraction methods. Metallographic inspection was carried out by sectioning the said sections vertically and polishing and examing the so-exposed face of each section. Metallographic examination of the uppermost horizontal section at a point about 0.3 centimeter from the top thereof revealed clusters of particles which were identified by X-ray diffraction as MgO. The matrix surrounding the magnesium oxide in these sections was identified as a magnesium-zinc alloy. Examination, by the metallographic technique, of sections taken from the middle portion of the solidified melt showed that this portion consisted mainly of magnesium-zinc alloy along with an incompletely characterized magnesium-zinc phase which may contain uranium. The lower portion of the solidified melt was found to consist of magnesium-zinc alloy, uranium-zinc intermetallic compound (mainly U2ZI117) and the incompletely characterized magnesiumzinc phase referred to above. Two sections, each 0.5 centimeter thick, were cut from the bottom end of the solidified melt and analyzed chemically. The analyses are as follows:

The results of the foregoing test made according to the invention show that MgO floats in the magnesium-zinc composition used while uranium-zinc intermetallic compounds settle.

Among the advantages of the invention are (l) the greater ease of reduction of uranium oxide and (2) the concurrent purification of the reduced uranium on employing magnesium-zinc alloys containing higher proportions of magnesium.

I claim:

1. In the process of preparing uranium metal from uranium oxide by contacting the uranium oxide under an inert atmosphere and at an elevated temperature with a molten magnesium-zinc mixture to form a reaction mixture and recovering the uranium thereby formed, the improved sequence of steps which comprises: contacting the uranium oxide with at least percent of the stoichiometric amount of a magnesium-zinc alloy containing from about 5 to 50 parts by weight of zinc per part of uranium oxide, expressed as uranium, for a time sufiicient for substantially all of said oxide to be reduced to uranium metal and for the uranium metal to alloy with said zinc; allowing the magnesium oxide formed in the reduction step to rise to the top portion of the reaction mixture and uranium-zinc intermetallic compound to settle; removing the top portion of the reaction mixture containing the magnesium oxide from the remainder of the reaction mixture; removing the bottom portion of the reaction mixture containing the settled uranium-zinc intermetallic compound from the remainder thereof; and heating said separated bottom portion under an inert atmosphere thereby distilling magnesium and zinc therefrom.

2. In the preparation of uranium-zinc alloy from uranium oxide by the process in which uranium oxide is heated together with at least 110 percent of the stoichiometric amount of molten magnesium thereby forming magnesium oxide and uranium-zinc alloy, the improved method of separating magnesium oxide from the reaction mixture which comprises: providing in the reaction mixture at least three parts of zinc per part of residual magnesium while the reaction mixture is at a temperature above the melting temperature of magnesium-zinc alloy so as to cause magnesium oxide present to rise to the top portion of the reaction mixture; and separating the top portion of the reaction mixture containing said magnesium oxide from the remainder of the reaction mixture while the uranium-zinc alloy is substantially precipitated.

3. In the preparation of uranium-zinc alloy from uranium oxide by the process in which titanium oxide is heated together with at least 110 percent of the stoichiometric amount of molten magnesium thereby forming magnesium oxide and uranium, the improved method of separating magnesium oxide from the reaction mixture which comprises: providing in the reaction mixture from about five to fifty parts by weight of zinc per part of uranium in the reaction mixture While the reaction mixture is at a temperature of at least 450 C. thereby forming uranium-zinc alloy; allowing the magnesium oxide present in the reaction mixture to rise to the top portion thereof; and separating the top portion of the mixture containing the magnesium oxide from the remainder of the reaction mixture while the uranium-Zinc alloy is substantially precipitated.

4. In the preparation of uranium-zinc alloy from uranium oxide by the process wherein the said uranium oxide is heated together with at least 110 percent of the stoichiometric amount of a magnesium-zinc mixture whereby the uranium oxide is reduced to uranium, magnesium oxide is formed, and the uranium alloys with the zinc, the improved method of separating the magnesium oxide from the reaction mixture which comprises: bringing the zinc content of the residual molten magnesiumzinc mixture to at least Weight percent; allowing the magnesium oxide in the reaction mixture to rise to the top portion thereof While the reaction mixture is molten; and separating the top portion of the reaction mixture containing the magnesium oxide from the remainder of the reaction mixture while the uranium-zinc alloy is substantially precipitated.

5. In the process of preparing uranium-zinc alloy from uranium oxide by contacting uranium-oxide under an inert atmosphere and at an elevated temperature with a molten magnesium-zinc mixture and recovering the uranium-zinc alloy thereby formed, the improved sequence of steps which comprises: contacting the uranium oxide with at least percent of the stoichiometric amount of a magnesium-zinc mixture containing from about five to fifty parts by weight of zinc per part of uranium oxide, expressed as uranium, for a time sufi'icient for substantially all of the uranium oxide to be reduced to uranium and alloyed with the zinc; allowing the magnesium oxide formed in the reduction process to rise to the top portion of the reaction mixture while the reaction mixture is molten; and separating the top portion of the reaction mixture containing the magnesium oxide from the remainder of the reaction mixture While the uranium-zinc alloy is substantially precipitated.

Marden Sept. 24, 11929 Knighton et a1 Apr. 26, 1960 

1. IN THE PROCESS OF PREPARING URANIUM METAL FROM URANIUM OXIDE BY CONACTING THE URANIUM OXIDE UNDER AN INERT ATMOSPHERE AND AT AN ELEVATED TEMPERATURE WITH A MOLTEN MAGNESIUM-ZINC MIXTURE TO FORM A REACTION MIXTURE AND RECOVERING THE URANIUM THEREBY FORMED, THE IMPROVED SEQUENCE OF STEPS WHICH COMPRISES: CONTACTING THE URANIUM OXIDE WITH AT LEAST 110 PERCENT OF THE STOICHIOMETRIC AOMUNT OF A MAGNESIUM-ZINC ALLOY CONTAINING FROM ABOUT 5 TO 50 PARTS BY WEIGHT OF ZINC PER PART OF URANIUM OXIDE, EXPRESSED AS URANIUM, FOR A TIME SUFFICIENT FOR SUBSTANTIALLY ALL OF SAID OXIDE TO BE REDUCED TO URANIUM METAL AND FOR THE URANIUM METAL TO ALLOY WITH SAID ZINC; ALLOWING THE MAGNESIUM OXIDE FORMED IN THE REDUCTION STEP TO RISE TO THE TOP PORTION OF THE REACTION MIXTURE AND URANIUM-ZINC INTERMETALLIC COMPOUND TO SETTLE; REMOVING THE TOP PORTION OF THE REACTION MIXTURE CONTAINING THE MAGNESIUM OXIDE FROM THE REMAINDER OF THE REACTION MIXTURE; REMOVING THE BOTTOM PORTION OF THE REACTION MIXTURE CONTAINING THE SETTLED URANIUM-ZINC INTERMETALLIC COMPOUND FROM THE REMAINDER THEREOF; AND HEATING SAID SEPARATED BOTTOM PORTION UNDER AN INERT ATMOSPHERE THEREBY DISTILLING MAGNESIUM AND ZINC THEREFROM. 